This invention relates to bending glass sheets by the gravity sag technique and particularly relates to a technique to be used to reduce the incidence of glass sheet breakage during a glass sheet bending operation. Glass sheets are usually bent by a gravity sagging technique in which a flat glass sheet is mounted on the sectionalized outline mold comprising a rigid metal rail disposed edgewise with its upper edge forming a surface that conforms in elevation and outline to the shape desired for the bent glass sheet. The metal rail must be sufficiently massive to maintain its shape while it supports the glass sheet at elevated temperature. Hence, the metal rail has a heat capacity per unit area greater than that of the glass sheet.
When glass sheets are shaped to relatively deep bends, the length of the chord between the ends of the bent glass is shorter than the distance between the ends of the flat glass sheet before bending. In order to reduce relative sliding between the glass ends and the metal rail which marks the glass, the mold rail comprises articulated rail sections including an end rail section whose contour conforms to the outline and shape desired for the longitudinal end portion of the glass sheet to be bent. The end rail section is constructed and arranged to pivot downward into a lower position to support the mass of a relatively stiff flat glass sheet to be bent, and when the glass sheet is softened by heat, to pivot upward into an upper position where it cooperates with additional shaping rails to form a substantially continuous outline shaping surface conforming in contour and elevation to the shape desired for the glass sheet. Making the mold rail of articulated rail sections instead of a continuous enclosed shaping rail increases the need for the rail to have greater cross-section to provide the needed structural rigidity at the glass softening temperature. Therefore, articulated metal shaping rails tend to have a heat capacity per unit area that differs from that of the supported glass sheet by an even greater amount than that of a continuous metal rail.
Depending on the shape to be imparted to the glass, the outline mold comprising articulated rail sections includes one or more end rail sections that pivot relative to an adjacent rail section between a lower position to support a flat glass sheet for bending and an upper position where it helps forms a substantially continuous outline shaping surface conforming to the shape desired for the bent glass. The end rail section is counterweighted about its pivot axis in such a manner that it tends to be in its lower position when the mold supports a flat rigid glass sheet and to pivot to its upper position when the mold supports the glass in heat-softened condition.
In order to enable the end rail section to pivot upward, the glass laden mold is subjected to a temperature cycle that comprises heating the latter to the glass softening temperature so that the end section rail pivots upward to impress its shape onto the heat-softened glass sheet while the sheet sags by gravity to conform to the shape of the mold. After the sheet is so shaped, the glass laden mold is cooled to minimize excessive sagging of the glass sheet within the outline of the mold. The shaped glass sheet so cooled is then removed from the mold, inspected and further processed.
The glass sagging technique has been the method used to bend two glass sheets simultaneously, which sheets are subsequently laminated together to form a laminated automobile windshield. The latter is curved to conform and blend into the shape of an automobile vehicle in which it is installed.
Outline glass sheet bending molds usually comprise relatively rigid massive shaping rails in the form of a T in cross-section inverted so as to support the edge of the bent glass sheet near the glass sheet periphery on the base of the stem of the T. Steel is ordinarily used for the shaping mold rails because steel rails maintain their shape throughout the temperature cycle needed for bending and annealing or bending and tempering, which temperature cycle involves a heating cycle in a bending lehr followed by a controlled cooling cycle. However, since the temperature of mold rails reacts more slowly to the change of temperature in the environment to which the glass laden molds are subjected than the glass, the glass portion in contact with the mold rail has its heating rate retarded compared to the heating rate of the portions of the glass sheet that are spaced from the metal shaping rail during the mass production bending of bent laminated windshields and other bent fabricated parts by the gravity sag technique. This establishes a thermal gradient in the glass between the shaping rail contacting portion and the glass sheet portion directly exposed to the hot atmosphere of the lehr that is steep enough to cause glass breakage.
It has been proposed to wind tapes or elongated strands twisted into cords of elongated insulating material such as fiber glass tapes and the like in order to insulate the glass sheet from direct contact with the metal shaping rail of the outline mold. However, such winding of the insulating tapes is a costly and time consuming process and since the tapes have a limited durability before they require replacement, such a solution does not appear feasible to reduce glass breakage incidental to the glass bending temperature cycle.
Another proposed solution is to apply localized heat between passes through the bending lehr to only those portions of the outline mold that contacts the glass throughout the heating cycle. This solution enables the flat glass contacting mold portions to enter the bending lehr at a sufficiently higher temperature than the glass loaded on the mold for bending so that the heat stored in the mold by the selective preheating helped to heat the local glass portions in contact with the selectively heated mold rail portions, thus reducing the steepness of the thermal gradient in the glass. However, steadily rising production rates made the time of transit of the glass laden molds through the bending and annealing lehr so short that it became impractical to heat the shaping rail portions sufficiently to have them retain enough heat to impart local heat to the glass sheet bending cycle that the resulting temperature gradient in the glass between the rail contacting portion and the remainder of the glass was sufficiently gradual to avoid the stresses that induce glass breakage during the bending. If the mold portions that contacted the glass initially were heated sufficiently to have a gradual thermal gradient at the glass softening temperature, the thermal gradient established after the glass was loaded on a mold with superheated flat glass contacting portions would be too steep to avoid glass breakage.